Electronic devices are becoming more prevalent and more complex. Consumer electronics, such as appliances, telephones, entertainment systems, etc., are present in virtually every home. Businesses have integrated electronic devices into offices and factories. Even traditionally mechanical devices, such as automobiles, are incorporating ever larger and more complex electronic elements. Among the most sophisticated and complex electronic devices are modern digital computer systems.
Electronic devices generate and are penetrated by electro-magnetic radiation. The frequencies and amplitudes of the radiation vary depending on the device. Sometimes such radiation is a necessary part of the normal function of the device, as in the case of a radio transmitter communicating information over a selected, tuned electro-magnetic frequency. In most cases, however, such radiation is an unwanted by-product of electronic activity taking place inside the device.
Problems associated with electro-magnetic radiation are particularly acute in the case of digital computing devices. A modern digital computing device typically comprises a large number of very small, active electronic circuit elements such as transistors, switching state and transmitting signals very rapidly. The number of such elements and high switching rate generate considerable electro-magnetic radiation. At the same time, the short time interval within which state switches must be accomplished, make the circuit elements particularly susceptible to stray electro-magnetic radiation generated by other devices. Such stray radiation can cause erroneous state switches, or prevent switches that should occur, resulting in corruption of the electronic signals and loss of data.
In recent years there has additionally been concern over the potential impact of such electro-magnetic radiation on human health. While definitive proof of the nature of any such impact is not yet available, various governmental entities have proposed or promulgated laws, rules or guidelines which limit the amount of radiation emitted by an electronic device. It is possible that as more information concerning the effect of electro-magnetic radiation becomes available, such limits will become more stringent.
Various techniques for shielding electronic devices are known in the art. For example, it is known that solid, continuous metal enclosures of sufficient thickness provide a good barrier to electro-magnetic radiation. However, such enclosures are costly, heavy, or may otherwise interfere with preferred design considerations. It is also known that a molded plastic enclosure having a conductive coating (usually a conductive paint) will provide some degree of protection. Although this solution is relatively lightweight, the coating process, including secondary surface preparation, can add considerable cost to the device, and it does not always provide as the desired degree of shielding.